This invention relates to an experimental plant for the single or multiple coating of webs of paper or foil with viscous solutions followed by drying. A plant of this type comprises at least one casting apparatus followed by a blow drier.
When designing a manufacturing plant, e.g. for the coating and drying of foils or boards, it is very important to know the optimum coating and drying conditions for producing the best quality product before the plant is designed.
It is also very valuable if a product which has been developed on laboratory scale can also be tested on a small scale for its behaviour under the expected manufacturing conditions and can be adjusted to these conditions. An exact knowledge of such data is all the more important the more the quality of the product or substance depends on the manufacturing conditions, e.g. the temperature during drying, the moisture, the quantity of air, etc. This applies to a particularly large extent to photographic products.
In German Auslegeschrift No. 1,962,089 there is described a multiple casting installation in which the supply of drying air is to a large extent variable. This installation is intended for production. It would be uneconomical to use it for experimental purposes on account of its size, but there is a limit to the extent to which its size can be reduced because a certain minimum length is required for the drying sections, so that even when reduced in size the installations are still relatively expensive.
An experimental casting machine for the single or multiple coating and drying of webs of paper or foil has been described in German Auslegeschrift No. 2,246,798. In this installation, the web of material moves in a loop and the individual casting apparatus with their stations are moved sideways towards the web to a position underneath the casting roller when they are to be used for casting, so that the supply and metering pipes which are necessary for modern coating processes must be moveable in order to move with the casting apparatus. This is a serious disadvantage, particularly when several casting apparatus are employed.
Moreover, in this experimental plant according to German Auslegeschrift No. 2,246,798, it is impossible to cover the web of material uniformly and completely with a blast of drying air in the region where the web is deflected over the casting roller. It is therefore impossible with this plant to achieve uninterrupted drying under specified conditions which enables one to draw conclusions applicable to the process carried out on a factory scale.
Another disadvantage of this installation is that constructional measures designed to eliminate harmful external influences (e.g. vibration of the building) which could interfere with the coating process, are difficult to carry out and then only at considerable expense. Although the whole machine could be mounted on vibration insulators, this would have the disadvantage that the resulting system, which would be insulated against the floor, would still contain within itself many potential sources of vibration (driving motors, metering devices, among others).
Another disadvantage which has shown up in practice is that if the web is moved over a drum of relatively large thermal capacity in controlled drying experiments, the result can be substantially affected by uncontrolled thermal effects from the reverse side.
Furthermore, when a web is moved along a path as shown in FIG. 1 of German Auslegeschrift No. 2,246,798, it is difficult to place the loop of web into position without damaging it, especially if it already contains sensitive layers and if for economical reasons the machine is operated by only one person.
Another disadvantage is that in order to ensure good contact with the casting roller, the web must be passed over a centre disc roll on the sensitive side of the material. It is found in practice that only relatively narrow webs can be treated in this way without deformation of the foil, especially if the foil is thin. Where it is necessary to use wider webs, for example for certain tests, this type of centre disc deflection cannot be employed. Replacement of the centre disc roll, for example by an air bearing or so-called "vacuum roller" would, however, require complicated technology and render the conducting of experiments more difficult.
In such an installation, the casting apparatus must be removed from the table for rinsing. The time required for this is in some cases too long to leave sufficient time to enable all the impurities to be thoroughly washed out of the casting apparatus (some modern photographic substances, e.g. instant hardener solutions, undergo very rapid chemical reactions).
Finally, it has been found in this installation that in the region of the deflecting drum it is impossible to carry out a controlled transfer of heat from the reverse side varying with time, e.g. by means of air blasts, as is often desired.